I would just get an SM420, that came with them when new. They are fairly easy to find and you will have no exotic problems getting the truck back on the road. You need to go through the casting number search to pinpoint what engine you have first. That is an easy search. Depending on your exact truck, the rear end upgrade, new gears, may be the easiest route to a little more speed in high gear.
If you have two engines that spin, do compression checks and possibly start them on a stand and see if they are any good. Everyone here seems to like the leak down tests as well. The engines will both most likely need some internal cleaning. My guess is that both are going to run and would work. I would tend to put the GMC back into the GMC Truck if possible.
I would also guess you have Huck Brakes. They too will work fine but require more care and maintenance than newer brakes. You just have to learn about them.
You have a lot of options.
